#app4 {}

@keyframes change {
    0% {
        background: red;
    }

    100% {
        background: purple;
    }
}

#app4 .circle {
    width: 25vw;
    height: 25vw;
    border-radius: 50%;
    border: 1px solid red;
}

#app4 .circle.active {
    animation: change 1.5s infinite alternate linear;
}